I have never hunted squirrel's with a dog. From what I've read over the years, one of the biggest assets of a dog is that it goes to the other side of the tree making the squirrel go around to your side giving you the shot.
I have been thinking about a squirrel dog myself lately, after "Otis" is gone. The Jack Russell has been my first concideration and a rat terrier has made me think. As long as they bark a bit probally either would do. Next door neighbor has a "Min Snou93874693099 (got no idea how to spell that)", that barks all the time and chase's every thing that moves on my place!